Industrial waste water from a plant or domestic waste water contains a large amount of organic matters, such as oils in the form of oil droplets or an emulsion and organic matters that are present in water in the form of ions or molecules. Industrial waste water containing organic matters is subjected to a purification treatment, such as the separation of oils and the decomposition and removal of organic matters by microorganisms. The treated water that has been subjected to a purification treatment is subjected to a demineralization treatment that removes ions contained in the waste water, and the treated water is reused as industrial water. In addition, organic matters are also contained in water in nature, such as rivers and lakes. Water taken from nature is also subjected to a purification treatment.
As demineralization treatment devices, reverse osmosis membrane demineralizers, capacitive de-ionization treatment devices (e.g., Patent Literature 1), and the like are known.
A reverse osmosis membrane demineralizer has a reverse osmosis membrane (RO membrane) inside. When water containing ions flows into a reverse osmosis membrane demineralizer flows, the reverse osmosis membrane (RO membrane) allows only water to permeate therethrough. The water that has permeated through the reverse osmosis membrane (treated water) is reused as industrial water, etc. On the upstream side of the reverse osmosis membrane, ions that were not allowed to pass through the reverse osmosis membrane are accumulated, and thus there is a concentrated water having concentrated ions. The concentrated water is discharged from the reverse osmosis membrane demineralizer, and thus discharged out of the system of the water treatment device 1.
In the case of a reverse osmosis membrane demineralizer, when the proportion of the treated water relative to the water flowing into the demineralizer is increased, the scale component concentration of the concentrated water becomes equal to or higher than the saturation solubility, resulting in the deposition of crystalline solids (scale). As substances that deposit as scale, calcium carbonate (CaCO3), gypsum (CaSO4), calcium fluoride (CaF2), and the like are known. For example, when the concentration of calcium carbonate in water is 275 mg/l at pH 7.3, this exceeds the saturation solubility, and thus scale is deposited. However, scale deposition does not occur within a short period of time, such as 10 minutes, after the saturation solubility is exceeded, and scale deposition occurs after standing for a long period of time, such as one day. In a reverse osmosis membrane demineralizer, ion components are continuously removed by the membrane. Therefore, during operation with high water recovery, the ion concentration on the concentrated water side is constantly high, and a concentration equal to or higher than the saturation solubility is maintained for a long period of time (one day or more). Accordingly, scale is deposited on the concentrated water side in the reverse osmosis membrane demineralizer.
FIGS. 4(a) to 4(c) are schematic diagrams of a capacitive de-ionization treatment device. The capacitive de-ionization treatment device 100 is configured to include a positive electrode 101 and a negative electrode 102, which are a pair of opposed porous electrodes, and a flow path 103 that allows water to flow between the electrodes. An anion exchange membrane 104 is installed on the flow-path-side surface of the positive electrode 101, and a cation exchange membrane 105 is installed on the flow-path-side surface of the negative electrode 102.
A demineralization treatment by the capacitive de-ionization treatment device 100 is performed by the following steps.
(Demineralization Step)
First, electrodes are energized so that the positive electrode 101 is positively charged and the negative electrode 102 is negatively charged. That is, voltages having polarities opposite to each other are applied to the positive electrode 101 and the negative electrode 102, respectively. When water flows through the flow path 103 between the energized electrodes, negative ions in water permeate through the anion exchange membrane 104 and are adsorbed to a porous portion 101a of the positive electrode 101, while positive ions permeate through the cation exchange membrane 105 and are adsorbed to a porous portion 102a of the negative electrode 102 (FIG. 4(a)). The water from which ions have been removed is, as a treated water, provided for recycling, etc.
(Reproduction Step)
After the elapse of a predetermined period of time, the electrodes are energized so that the positive electrode 101 is negatively charged and the negative electrode 102 is positively charged. That is, voltages that are reverse to the voltages at the time of the adsorption of ions to the electrodes are applied to the positive electrode 101 and the negative electrode 102. As a result, the adsorbed ions are released from the positive electrode 101 and the negative electrode 102 and return to the flow path 103 (FIG. 4(b)).
After the released ions are sufficiently accumulated in the flow path 103 or at the same time as the release of ions, water is supplied to the flow path 103. Accordingly, water containing ions is discharged from the flow path 103, and the positive electrode 101 and the negative electrode 102 are regenerated to the state where no ions are adsorbed (FIG. 4(c)). The discharged water is recovered as a concentrated water.
Even if the saturation solubility is exceeded in the above regeneration step, when the regeneration step is performed within a short period of time, such as 10 minutes or less, the demineralization step starts before scale deposition, and the concentration becomes lower than the saturation solubility, whereby scale deposition is prevented. Because of this characteristic, a capacitive de-ionization treatment device can achieve higher water recovery (recovery of recyclable water) than a reverse osmosis membrane demineralizer and thus is more advantageous.
In order to increase the amount of ions adsorbed to electrodes in a capacitive de-ionization treatment device, larger electrode surfaces are more desirable. Therefore, a porous material made mainly of carbon having a large surface area per volume, such as activated carbon, is used for the electrodes.